Since the discovery of water photolysis using a semiconductor photoelectrode, i.e., the so-called Honda-Fujishima effect, much research on water decomposition using a photocatalyst has been conducted based on the idea that it may be an effective means for converting light into chemical energy. The following is known about the mechanism of water photolysis using a semiconductor photocatalyst. When an n-type semiconductor, for example, is used as a photocatalyst and light having energy greater than the band gap energy of the semiconductor is irradiated, electrons in the valence band are photo-excited into the conduction band to generate free electrons in the conduction band, and, in contrast, positive holes are generated in the valence band. A photocatalysis reaction then proceeds when a reduction reaction and an oxidation reaction are caused by the generated electrons and positive holes, respectively.
A semiconductor photocatalyst can cause water photolysis under conditions where the band width of the semiconductor is larger than the electrolytic potential (theoretical value 1.23 V) of water. In addition, the electrons in the conduction band must be able to reduce water and the positive holes in the valence band must be able to oxidize water. More specifically, the lower end of the conduction band must be located at the minus side with respect to the potential at which hydrogen is generated from the water, and the upper end of the valence band should be located at the plus side with respect to the oxygen generation potential.
As semiconductors which meet such conditions, titanium dioxide, strontium titanate, barium titanate, sodium titanate, cadmium sulfide, zirconium dioxide, iron oxide, etc., have been found. Moreover, it is known that the semiconductors that support a metal, such as platinum, palladium, rhodium, ruthenum, etc., as a promoter can be effectively used as a photocatalyst for water photolysis.
For example, reference No. 1 (Japanese Unexamined Patent Publication No. 11 (1999)-188269) and reference No. 2 (Japanese Unexamined Patent Publication No. 2000-126761) disclose examples using photocatalysts as mentioned above. These references disclose floating a porous material supporting a photocatalyst on the surface of a pond or the like, causing photocatalysis by irradiating the porous material with light, and thus purifying the water.
If sunlight can be used for water photolysis, the hydrogen and oxygen generated can be stored and used to obtain heat and electricity through reactions, when required. In other words, sunlight energy can be converted into chemical energy and stored, thus providing an extremely effective method of utilizing solar energy.
However, catalysts, in particular, metal catalysts, which actively generate hydrogen, also actively react hydrogen and oxygen, which is a problem because the water photolysis will thus cause a reverse reaction. For example, when a photocatalyst supporting platinum (Pt) is suspended in water and the suspension is irradiated with light, the hydrogen and oxygen which are generated through photolysis will mix before they leave the catalyst in the form of separate bubbles. The mixed hydrogen and oxygen thus contact and react with the Pt and return to water again, so only a small amount of hydrogen and oxygen can be obtained.
In order to solve this problem, reference No. 3 (Front page, Volume 33, No. 2, 45 to 58 pages, 1995), for example, discloses a process for increasing the contact between sunlight and a catalyst by dispersing powdery semiconductor photocatalysts in water and shaking the entire reaction apparatus. Reference No. 4 (Japanese Patent No. 3096728) discloses placing a photocatalyst on a water-absorbing material, and dampening the surface by impregnating the water-absorbing material with water, then irradiating the surface with sunlight from above.
However, since the process of reference No. 3 requires the use of mechanical energy, the amount of energy used to generate hydrogen is greater than the amount of energy that is obtained. In order to solve this problem, reference No. 4 has a structure such that water is supplied to the photocatalyst surface from the water-absorbing material and sunlight directly reaches the interface between the photocatalyst and water, which eliminates the need for mechanical mixture, such as shaking. However, in this structure, the photocatalyst disperses only on the surface of the water-absorbing material and the density is low, so it is difficult to obtain sufficient results.
There is also a problem in that sunlight has a wide energy distribution range, from the ultraviolet to the infrared spectral region, but only the energy from the ultraviolet to the visible light spectral region is used for water photolysis using a photocatalyst. Therefore, the solar energy in the infrared spectral region is not conventionally used. Accordingly, it cannot be said that the conventional photolysis system uses sunlight effectively.
The present invention was made in view of the above-described problems. It is an object of the present invention to provide a water photolysis system and process that can efficiently obtain hydrogen and oxygen by inhibiting reverse reaction, and, moreover, that can utilize solar energy effectively for promoting water photolysis.